Ecological thresholds of hypoxia and sedimentary HS in coastal soft-bottom habitats: A macroinvertebrate-based assessment.

Ecological thresholds of dissolved oxygen (DO) and sedimentary hydrogen sulfide (HS) for macrozoobenthos were examined during a 30-month monitoring of two stations in a highly eutrophic canal in inner Tokyo Bay, Japan. Bottom DO and HS concentrations fluctuated seasonally, and were significantly correlated with water and sediment temperatures. Red tide-derived phytodetritus was a major source of sediment organic matters in the canal bottom, and the sediment became highly reduced and sulfidic condition in warmer months (sedimentary HS; up to 8.5 mM). Dominant opportunistic taxa, including polychaetes and amphipods, were eliminated under low DO and high HS conditions (i.e., population thresholds), and devastation of community structure occurred at 2.4-3.3 mg l DO and 1.8-2.7 mM HS (i.e., community thresholds). To maintain ecosystem function in anthropogenically degraded habitats and ensure colonization by macrozoobenthos throughout the year, DO and HS levels should be maintained below these thresholds.